Ziegler-Natta type catalysts for the polymerization of olefins are well known. The traditional Ziegler-Natta type soluble systems comprise a metal halide activated to a catalyst species by reaction with a metal alkyl cocatalyst, particularly an aluminum alkyl cocatalyst. The activation of these traditional heterogeneous Ziegler-Natta catalysts generates a variety of different active sites. As a consequence of this non-uniformity of active sites, the catalysts produce polymer products having a broad molecular weight distribution (MWD). Furthermore, the polymer products exhibit broad composition distribution (CD), poor comonomer incorporation and block sequence distribution.
Catalysts formed when a bis(cyclopentadienyl) compound of the Group 4 metals, including zirconium and hafnium, is activated by an alumoxane, i.e., metallocene-alumoxane catalysts, whether homogeneous or supported, generally possess higher activity and are more versatile than conventional Ziegler-Natta type catalysts. These catalysts are part of a broader category of single-site catalysts, which have been used to produce a variety of polymer products including, for example, high density linear polyethylene (HDPE), linear low density polyethylene (LLDPE), ethylene-propylene copolymer (EP), non-crystalline polypropylene and crystalline polypropylene. Metallocene-alumoxane and other single-site catalysts offer the advantage over traditional Ziegler-Natta catalysts of being able to produce polymers with narrower MWD. Nonetheless, metallocene-alumoxane catalysts have limitations in practical commercial applications. Aluminoxane activators are relatively expensive because usually a substantial excess of aluminoxane to metallocene is required. Aluminoxane is also air sensitive/water reactive and is challenging to handle due to tendency to gel. Furthermore, metallocene-aluminoxane catalyst, while producing a relatively narrow MWD polymer product, has limited capability to produce high molecular weight polymers or polymers having a high comonomer content.
European patent applications 88300698.3 and 88300699.1, published in 1988 under publication numbers 277,003 and 277,004, respectively, describe ionic single-site catalysts with non-aluminoxane activators for activating bis(cyclopentadienyl)-substituted Group 4 metal based metallocenes. The activator is described in 277,003 as comprising a cation capable of donating a proton and reacting irreversibly with a ligand of the metallocene to liberate a free, neutral by-product and a compatible noncoordinating anion comprising a plurality of boron atoms, which compatible noncoordinating anion is stable, bulky and labile. The activator is described in 277,004 as an ion-exchange compound comprising a cation that will irreversibly react with at least one ligand of the metallocene and an anion that is a single coordination complex comprising a plurality of lipophilic radicals covalently coordinated to and shielding a central formally charge-bearing metal or metalloid atom, which anion is bulky, labile and stable to any reaction involving the cation of the second component. U.S. Pat. No. 5,153,157 describes similar catalyst systems with an additive that removes impurities and gives examples of such systems based on perfluoro groups.
An article published in Organometallics 2000, 19, 1625-1627, “Al-, Nb-, and Ta-Based Perfluoroaryloxide Anions as Cocatalysts for Metallocene-Mediated Ziegler-Natta Olefin Polymerization” by Yimin Sun, Matthew V. Metz, Charlotte L. Stern, and Tobin J. Marks, reports on ionic activator compound based on the pentafluorophenoxide group C6F5O−. This article also illustrates how single-site catalysts based on such activator compounds deactivate rather easily because pentafluorophenoxide group tends to be easily withdrawn from the activator due to interaction with the metallocene.
The systems based on perfluoro groups appear to rely on these groups as electron-withdrawing groups for stabilization. This has a stabilizing effect. As used herein, the term “electron-withdrawing” means capable of reducing electron density on a reaction center and the term “electron-donating” means capable of increasing electron density on a reaction center. Catalysts based on perfluoro groups tend to be relatively expensive. Another technique used for stabilization is chelating ligands. US Publication Number 2006/0009596 describes a borate with chelating ligands used for activating a single-site catalyst; yet the catalyst systems described must be fluorinated, preferably perfluorinated.
In spite of published descriptions of ionic single-site catalyst systems, it is observed that most of these systems are still based on relatively expensive perfluoro groups. A need exists for commercially feasible ionic, single-site catalyst activators that permit suitable control of molecular weight and molecular weight distribution in polymers and at the same time remain intact, without significant deactivation, for commercially acceptable periods of time.